The "Dirty Dozen" in Aviation Maintenance


Due to a spate of maintenance-related aviation incidents and accidents in the late 1980s and early 1990s, Transport Canada, together with the aviation industry identified 12 human factors, christened the “dirty dozen” which were human factors elements that degrade people’s ability to perform effectively and safely which could lead to maintenance errors. In 1994, dirty dozen posters were developed which provided information and guidance to maintenance personnel all over the world to identify and prevent these dirty dozen factors. Safety nets were also introduced so that the appropriate mechanisms can be put into place to prevent human errors. [1]

Why human factors in maintenance?

Studies and analysis have shown that maintenance is a large contributing factor in accidents. For example:

  • Sears conducted a detailed study of 93 major world-wide accidents between 1959 and 1983 and found that 12% of the accidents were attributed to maintenance and inspection factors (4th on the list). [2]
  • In an analysis done by Boeing in 1995, 15% of commercial accidents between 1982 and 1991 had maintenance as a large contributing factor. [3]

In an aircraft hangar, a typical maintenance environment would involve a myriad of different and difficult tasks simultaneously going on whilst the personnel are working on an aircraft. Numerous HF issues involved would include maintenance personnel having to work on aircraft structures in confined spaces or at high heights with coordination needed amongst various team members essential. These factors coupled with a high noise environment in the late hours of the night whilst having to make sure their jobs are meticulously done contribute to a high probability of mistakes and hence accidents.

Cost of not adhering to proper maintenance guidelines

The cost of not adhering to proper maintenance guidelines is heavy and avoidable and three findings in an article titled "Human Factors: "Beyond the Dirty Dozen" [4] can attest to that;

  • According to FAA, about 80 percent of maintenance mistakes involve human factors (HF) and if not detected, would lead to accidents.
  • According to researcher Alan Hobbs, in an Australian Transport Safety Bureau report, the cost of canceling a Boeing 747-400 flight can cost an airline $140,000, with delays at the gate costing $17,000 per hour off company’s revenues thus depicting the high costs of delays and flight cancellations.
  • The FAA manual also states that ramp incidents wreaked $5-billion-worth of damage worldwide in 2004.

Table of "Dirty Dozen" Human Factors

The table below provides a comprehensive description of the 12 Dirty Dozen Factors through the posters as well as providing the relevant safety nets that should be subscribed to.

The Dirty Dozen (All posters embedded from MARSS on 1 September 2009) Explanation Safety nets available
D1.gif Caption says "I guess day shift can finish screwing on the panel." Lack of communication is depicted as the worker is going to leave the panel of an aircraft unfinished without communicating this to the next worker in any written or verbal form, assuming that the next worker knows which part of the aircraft is left unfinished. Use logbooks to communicate, to remove doubt. Discuss work to be done and what has been completed to the one taking over from you. Never assume anything, always check.
D2.gif Caption says "I've looked back there 1,000 times and never found anything wrong." Complacency is depicted whereby a maintenance personnel has finished signing an inspection sheet whilst telling himself that he has checked at the aircraft part a thousand times without ever finding anything wrong; This is despite the fact that a component of the aircraft has a spoilt cable. Hence, by performing a routine task over and over again, overconfidence due to increasing proficiency could lead lead to complacency setting in and errors in judgment can appear. Train yourself to expect to find a fault and to consistently look out for these faults or hazards. Never sign for anything you didn't do. Never assume anything, always check.
D3.gif Caption says "This is the third one to bend! What's going on?" Lack of knowledge is depicted in which a maintenance personnel cannot fathom how a helicopter part could be bent for a third time even though he has kept working on it, demonstrating that there is a possibility there is a lack of certain type of knowledge in how he is remedying the helicopter part. This could be also due the rapid pace at which technology is evolving and hence increased knowledge is needed when performing the task. Don't rely on memory and consult the relevant up to date manuals. Always ask if you are in doubt. Get training on type.
D4.gif Caption says "Hey, Your wife is on the phone." Distraction is depicted as the personnel is informed of an urgent call from his wife, hence taking him away from focusing on his job which could potentially lead to errors later as his concentration has been affected. Always finish the job or unfasten the connection. Document and handover the uncompleted work. When you return to the job, re-inspect by another or always go back three steps by yourself. Use a detailed check sheet.
D5.gif Caption says "I thought you wanted him to turn left right here!" Lack of teamwork in which two marshallers are trying to guide an aircraft in opposite directions shows a fundamental lack of cooperation and communication which are essential in performing certain difficult tasks especially when more people are involved. Discuss what, who and how a job is to be done. Be sure that everyone understands and agrees through good communications and co-ordination with team members. Look out for one another.
D6.gif Caption says "I'm glad this double shift is over." Fatigue is depicted with a maintenance personnel blissfully unaware that he is reaching the end of the horizontal stabilizer as he has become fatigued after working a double shift. Be aware of the symptoms and look for them in yourself and others. Plan to avoid complex tasks when you are physically exhausted. Sleep and exercise regularly. Ask others to check your work. If you are fatigued, take a break.
D7.gif Caption says "We have nil stock of left skids so I guess this will have to do." Lack of resources is depicted with a maintenance personnel standing in front of a helicopter with two different components; a float on the left and a skid on the right; obviously one of the essential parts is insufficient and he has fixed a totally different part on and still wants the helicopter to continue to fly. Hence, there are times when there a lack of resources, a decision must be made not to fly for safety’s sake. Check suspect areas at the beginning of the inspection. Order and stock anticipated parts before they are required. Know all available parts sources and arrange for pooling or loaning. Maintain a standard and if in doubt ground the aircraft. Preserve all equipment through proper maintenance.
D8.gif Caption says "Hurry up or we're going to be late again!" Pressure is depicted with an aircraft captain who is averse to being late, rushing a maintenance personnel to finish closing up a panel and whilst doing so, he misses seeing a component jutting out. Be sure the pressure isn't self-induced. Communicate your concerns. Request for assistance. Just say No.
D9.gif Caption says "Listen, I own the aircraft and I say it's not a bad leak!" Lack of assertiveness is shown with the maintenance personnel being badgered by the owner of the aircraft and not asserting himself and backing down by telling him that the plane is indeed suffering from a bad oil leak. Hence by allowing himself to be intimidated and not speaking up, it could potentially lead to an accident. Be assertive and provide clear feedback when danger is perceived. If it's not critical, record it in the journey log book and only sign for what is serviceable. Refuse to compromise your standards. Allow team members to give their opinions and accept criticisms positively
D10.gif Caption says "We lost our best aircraft! How are they going to pay my wages? What if I'm sued?" Stress is depicted with a maintenance personnel pulling his cart of tools towards the moving propeller and his overstressed at having lost his best aircraft and the fact that he might be sued. Hence, the stress build up has exceeded his limits and he can’t think rationally and perform his task right. Be wary of the effects of stress on your performance. Stop and look rationally at the problem. Determine a rational course of action and follow it. Take time off or at least have a short break. Discuss it with someone. Ask fellow workers to monitor your work. Exercise your body. Ensure sufficient rest at all times
D11.gif Caption says "All the regulation said was, 'Install Where it is Easily Accessible." Lack of awareness is depicted as an aftermath in this incident whereby a passenger has hit his head against a fire extinguisher during an emergency as the maintenance personnel who had installed the fire extinguisher there showed a lack of awareness by not using common sense and vigilance by putting it at a place that though easily accessible could potentially hurt someone. Think of what may occur in the event of an accident. Check to see if your work will conflict with an existing modification or repair. Ask others if they can see any problem with the work done by checking periodically to ensure correct work procedures.
D12.gif Caption says "Never mind the Maintenance Manual. Its quicker the way we do it here." Norms are depicted with a forklift used to install a jet engine demonstrating how this procedure was conducted at this maintenance facility, that even though it is a flawed manner has become a norm as the majority performs in this way. Always comply with defined work procedures. Be aware that "norms" don't make it right.

Accidents in which Dirty Dozen have contributed

Crash of Alaska Airlines Flight 261
This page provides a synopsis of the crash of Alaska Airlines Flight 261 and how dirty dozen factors have contributed to it.
Crash of Air Midwest Flight 5481
This page provides a synopsis of the crash of Air Midwest Flight 5481 and how dirty dozen factors have contributed to it.


The dirty dozen factors have long since been identified but yet has can be seen from the examples of accidents it has contributed above, it has still not been fully eradicated in the realm of aviation maintenance. It is imperative that maintenance organizations inculcate a working environment that reiterates these factors holistically so that they are ingrained in the maintenance personnel; this can be done through human factors training workshops by reiterating these factors and showing the aviation accidents they have caused.

It is by creating a culture in which all personnel are aware of these factors and the safety nets involved, this would help decrease the high percentage of human factors involved in maintenance issues.

1. Dupont, G. (1997). The dirty dozen errors in aviation maintenance. In Meeting Proceedings Eleventh Federal Aviation Administration Meeting on Human Factors Issues in Aircraft Maintenance and Inspection: Human error in aviation maintenance (pp. 45-49). Washington, DC: Federal Aviation Administration/Office of Aviation Medicine.
2. Sears, R.L. (1986). A new look at accident contributions and the implications of operational training programmes (unpublished report). Quoted in Graeber and Marx: Reducing Human Error in Aviation Maintenance Operations. (presented at the Flight Safety Foundation 46th Annual International Air Safety Seminar, Kuala Lumpur, Malaysia, 1993)
3. Graeber, R.C. and Marx, D.A. (1993).: Reducing Human Error in Aviation Maintenance Operations. (presented at the Flight Safety Foundation 46th Annual International Air safety Seminar, Kuala Lumpur, Malaysia, 1993)
4. Charlotte Adams. (2009). Human Factors: Beyond the "Dirty Dozen". Retrieved from the World Wide Web on 4 Sep 2009

Want to know more?

Human Factors: Avoid the Dirty Dozen with Safety Nets.
This article extols the virtues of avoiding the dirty dozen through safety nets by human factors training.
MRM Handbook
It provides a good write up on the need for human factors to be dealt with in a maintenace context and the differences between MRM and CRM.
The "Dirty Dozen" in ASRS Maintenance Reporting
It provides good accounts of how "dirty dozen” human factors play a role in maintenance incidents reported to the ASRS.

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